Apertured ball valves are well known for controlling the flow of a fluid through a bore, particularly in the oil and chemical process industries. Ball valves are compact, inexpensive and relatively easy to operate.
In an apertured ball valve, the valve operation may be broken down into two separate stages; firstly, the ball moves between an open and a closed position by rotating through 90° such that the ball aperture moves from an orientation coaxial with the flow direction, i.e. when the valve is open, to a position whereby the ball aperture is perpendicular to the flow direction. Secondly, the valve seals in the closed position to prevent flow through the bore across the ball valve.
Conventional ball valves will typically incorporate a seal made of a softer material such as polyetheretherketone (PEEK) or polytetrafluoroethylene (PTFE). However, these materials are vulnerable to degradation over the course of time, and consequently the valve may become unreliable.
In situations requiring the highest sealing integrity to be maintained, metal sealing valves such as gate valves are typically used. These valves are generally larger and more expensive than a ball valve of the equivalent bore size and pressure rating. In certain situations, such as wellhead and in-riser applications, it may be impossible to incorporate a gate valve to provide a metal seal due to the envelope restrictions. In these situations it is common to use a plug type device which incorporates a metal-to-metal seal on which is deployed wireline or coiled tubing. This method of deployment is cumbersome, time consuming and expensive.
Further drawbacks associated with ball valves include problems of distortion of the ball under high pressure due to the presence of a bore through the ball. This distortion can prevent a high integrity seal from being formed between the ball element and the valve housing.